Optical member and method of producing the same, and projection aligner

ABSTRACT

An optical member of the invention is an optical member for photolithography used together with light having a wavelength of not more than 250 nm. The optical member is made of a fluoride crystal in which a maximum diameter d max  (cm) of scattering bodies existing internally and a quantity n s  of the scattering bodies per 1 cm 3  satisfy a condition represented by following formula (1):  
     0&lt; d   max   2   ×n   s &lt;6.5×10 −4  (cm −1 )  (1)

FIELD OF THE INVENTION

[0001] The present invention relates to optical members made of fluoridecrystalline materials for use in ultraviolet and vacuum-ultravioletregions with a wavelength of not more than 250 nm, a method ofmanufacturing the same, and projection exposure systems such as steppersor scanners which apply the optical members to optical systems.

BACKGROUND ART

[0002] In recent years, micro-fabrication technology on wafers has beenrequested along with development of higher integration and highersophistication in VLSI (very large scale integration), andphotolithographic technology has been widely used as a fabricationmethod therein. It is desirable that a projection lens of a projectionexposure system, which is a key to the photolithographic technology,possesses high imaging performances (resolution and focal depth).

[0003] The resolution and the focal depth depend on a wavelength oflight and on an NA (numerical aperture) of a lens which are used uponexposure. When an exposure wavelength λ is constant, an angle ofdiffraction light grows larger as a pattern becomes finer. Accordingly,it is not possible to pick up the diffraction light unless the NA of thelens is large. Meanwhile, when the exposure wavelength λ is short, theangle of the diffraction light in the same pattern becomes small.Accordingly, the NA of the lens can be made small.

[0004] The resolution and the focal depth are severally represented bythe following formulae (2) and (3), namely,

Resolution=k ₁ ·λ/NA  (2)

Focal depth=k ₂·λ/(NA)²  (3)

[0005] (in the formulae (2) and (3), k₁ and k₂ severally denoteproportionality constants).

[0006] From the formula (2), it is learned that either the NA of thelens should be increased (that is, a diameter of the lens is increased)or the exposure wavelength λ should be shortened in order to enhance theresolution, however, it can be said from the formula (3) that shorteningλ is particularly advantageous from the viewpoint of the focal depth.

[0007] Due to the foregoing reason, recently the exposure wavelength isgradually shortened, and a projection exposure system adopting a lightsource such as a KrF excimer laser (248-nm wavelength) or an ArF excimerlaser (193-nm wavelength) is launched in the market. In such a system,optical materials usable for photolithography with a wavelength at 250nm or less are extremely limited, and almost all optical systems aredesigned to use two types of materials which are calcium fluoride andfused silica.

[0008] Moreover, practical application of a projection exposure systemusing an F₂ laser (157-nm wavelength) is under consideration in order toexploit further shortening of the exposure wavelength. It is consideredthat materials usable under this wavelength are limited to certainfluoride crystals beside calcium fluoride, which are strontium fluoride,barium fluoride, lithium fluoride and the like.

[0009] Incidentally, it is known that an absorption band is generated ina single crystal of calcium fluoride when a laser beam with high photonenergy is irradiated onto the single crystal of calcium fluoridecontaining a large degree of impurities. If an optical member using sucha material is applied to an optical system, there is a case of incurringdeterioration of transmittance in the used wavelength under theinfluence of the absorption band thus generated. Therefore, there isdisclosed a use of a single crystal of calcium fluoride having hightransmittance and durability with respect to an irradiated laser beam ascalcium fluoride for photolithography (Japanese Patent Laid-Open No.11(1999)-60382 and the like).

[0010] Next, speaking of an increase in the diameter of the lens, anoptical material for use in photolithography which requires extremelyhigh-level imaging performance is not only satisfied with a largediameter, but it is also essential that the optical material possessessmall birefringence and excellent homogeneity in an internal refractiveindex.

[0011] The Bridgman method is generally used as the method ofmanufacturing calcium fluoride. Upon fabrication of an optical memberfrom a calcium fluoride crystal ingot obtained by the Bridgman method,although there is a case of carving the optical member (material) of atargeted size directly out of the ingot, there is also a case of cuttingthe ingot into a plurality of blocks and then subjecting the blocks to aheat treating step so as to enhance internal quality such asbirefringence and homogeneity in refractive index. For example, JapanesePatent Laid-Open No. 11(1999)-240798 discloses a method of manufacturinga single crystal of calcium fluoride, which has characteristics ofbirefringence in the direction of the optical axis at 2 nm/cm or below,birefringence in the lateral direction (the direction of an in-planediameter perpendicular to the optical axis) at 5 nm/cm or below, and arefractive index difference An at 2×10⁻⁶ or below.

DISCLOSURE OF THE INVENTION

[0012] As described above, transmittance and durability with respect toa laser beam, as well as low birefringence and homogeneity in refractiveindex are required from the optical member. Particularly, the opticalsystem of the projection exposure system used for, photolithographyenhances its resolution to the limit. Accordingly, it is usual that theoptical system includes a substantial number of lenses for correction ofwave front aberration and therefore has a long optical path length.Here, even if an amount of transmission loss (an amount of loss byscattering+an amount of loss by absorption) of a lens is minute,accumulation of such transmission losses on the whole optical systemlargely affects the optical performance of the system. For example, inan optical path length of 1 m (=100 cm), when the amount of transmissionloss is only 0.5%/cm, intensity of light is reduced by 0.995¹⁰⁰° 0.606,that is, down to about 61% in the end. Accordingly, it is more favorableas internal transmittance comes closer to 100%/cm regarding the opticalmember used therein, and the internal transmittance is required to be atleast 99.5%/cm and above, or preferably 99.8%/cm and above.

[0013] Now, an index for judging quality of a fluoride crystallinematerial such as calcium fluoride in terms of transmittance is so calledinclusion, which is presence of defects inside the material. Althoughdefinition of the inclusion is not always clear, one aspect of theinclusion is observed as a luminous grain caused by light scattering inthe event of watching under a spotlight. Such a grain is called ascattering body.

[0014] Here, if the scattering body exists in the material, thescattering body scatters the light and thereby deterioratestransmittance. Accordingly, in the case of a system composed of numerousoptical members (lenses) such as an optical system used forphotolithography, even if an optical member sufficient in thecharacteristic such as durability of transmittance with respect to alaser, an amount of birefringence or homogeneity in refractive index,there are possibilities of adverse influences such as insufficiency inthroughput of the entire optical system, deterioration in contrast,occurrence of flares and ghosts, or the like.

[0015] For this reason, a fluoride crystalline material including thescattering bodies which are easily visible to the naked eye all over itssurface is deemed as a defective product. Moreover, in a case of afluoride crystalline material including local distribution of thescattering bodies, an optical member 3 with a small diameter, which iscarved out of a portion 2 b without the scattering bodies, is only usedfor the optical system as shown in FIG. 2. Here, the remaining portioninclusive of the scattering bodies 2 a is deemed as a defective product.

[0016] In this way, the scattering body is a large factor in degradationof optical characteristics and a product yield of calcium fluoride forphotolithography. Therefore, calcium fluoride as an optical member forphotolithography and a projection exposure system using the calciumfluoride are extremely expensive.

[0017] Moreover, it is possible to carve out the portion without thescattering bodies selectively in the case of the optical member with asmall diameter as described above. However, since the scattering bodiesare included upon carving out an optical member of a large diameter (adiameter φ at 200 mm, for example), it is extremely difficult to achieveenhancement in the optical characteristics and an increase in thediameter of the optical member at the same time.

[0018] The present invention has been made in consideration of theabove-mentioned problems of the prior art. It is an object of thepresent invention to provide an optical member which has sufficientlyhigh optical characteristics (such as internal transmittance) withrespect to light with a wavelength of not more than 250 nm andeffectuates enhancement in a yield and an increase in a diameter uponcarving out of a fluoride crystalline material, a method ofmanufacturing the same, and a projection exposure system using theoptical member.

[0019] To attain the foregoing object, the inventors of the presentinvention has extended studies on quantitative relations between opticalperformances required for a fluoride crystalline material (such ascalcium fluoride) usable as an optical member for photolithography andscattering bodies on the inside to begin with. As a result, theinventors have found out that an optical member carved out of a fluoridecrystalline material is usable as an optical member for photolithographyin spite of presence of scattering bodies therein in the case when thesize and the number thereof satisfy given conditions, and have reachedconsummation of the present invention.

[0020] Specifically, an optical member of the present invention is anoptical member for photolithography used together with light having awavelength of not more than 250 nm, consisting essentially of a fluoridecrystal in which a maximum diameter d_(max) (cm) of scattering bodiesexisting internally and a quantity n_(s) of the scattering bodies per 1cm³ satisfy a condition represented by the following formula (1):

0<d _(max) ² ×n _(s)<6.5×10⁻⁴(cm⁻¹)  (1)

[0021] According to the optical member of the present invention, themaximum diameter d_(max) of the scattering bodies and the quantity n_(s)thereof per 1 cm³ satisfy the condition represented by the foregoingformula (1), whereby optical performance (such as internaltransmittance) with respect to the light having the wavelength of notmore than 250 nm are maintained at a high level. Therefore, it ispossible to enhance a yield upon carving out of the fluoride crystallinematerial and to increase a diameter thereof.

[0022] Note that the scattering body relevant to the present inventionrefers to an object which exists inside the optical member and isobserved as a luminous grain caused by light scattering in the event ofwatching under a spotlight. To be more precise, the scattering bodiesinclude vacuum or air bubbles, and impurities such as graphite orcalcium oxide. These scattering bodies seldom have spherical shapes butgenerally have angular shapes.

[0023] Moreover, in the optical member of the present invention, it ispreferable that the maximum diameter of the scattering bodies is notmore than 2.0×10⁻³ cm (20 μm) and the quantity of the scattering bodiesper 1 cm³ is not more than 160, or alternatively, that the maximumdiameter of the scattering bodies is not more than 4.0×10⁻³ cm (40 μm)and the quantity of the scattering bodies per 1 cm³ is not more than 40.If these conditions are satisfied, it is possible to further enhance theoptical performance such as the internal transmittance.

[0024] Moreover, it is preferable that the optical member of the presentinvention has a diameter φ of not less than 200 mm. Since the opticalmember of the present invention possesses the high-level opticalperformance as described previously, when it is feasible to increase thediameter as large as the diameter φ of 200 mm, it is possible to furtherenhance an imaging performance in photolithography using the lighthaving the wavelength of not more than 250 nm.

[0025] Moreover, in the optical member of the present invention, it isseverally preferable that an amount of birefringence in a direction ofan optical axis is 2 nm/cm or below, that an amount of birefringence ina diametrical direction is not more than 5 nm/cm and that a refractiveindex difference An inside the member is not more than 2×10⁻⁶. If theseconditions are satisfied, it is possible to further enhance the imagingperformance in the photolithography using the light having thewavelength of not more than 250 nm.

[0026] Moreover, in the optical member of the present invention, it ispreferable that an amount of transmittance deterioration in a case ofirradiating 10⁶ pulses of an ArF excimer laser beam having energydensity of 50 mJ/cm²/pulse is not more than 2.0%/cm. If such a conditionis satisfied, it is possible to further enhance the imaging performancein the photolithography using the light having the wavelength of notmore than 250 nm.

[0027] Moreover, a method of manufacturing an optical member of thepresent invention includes a crystal growing step of melting a mixtureof fluoride powder and a scavenger at a melting temperature of a meltingpoint of the fluoride and above and then crystallizing the melted fluidand further cooling down an obtained fluoride crystal in a temperaturerange from 1000° C. to 900° C. by a temperature decreasing rate in arange from 0.1 to 5° C./hr, and a carving step of carving an opticalmember out of the fluoride crystal obtained in the crystal growing stepsuch that the optical member is made of a fluoride crystal in which amaximum diameter d_(max) (cm) of scattering bodies existing internallyand a quantity n_(s) of the scattering bodies per 1 cm³ satisfy acondition represented by the following formula (1):

0<d _(max) ² ×n _(s)<6.5×10⁻⁴ (cm⁻¹)  (1)

[0028] According to the manufacturing method of the present invention,the mixture of the fluoride powder and the scavenger is melted at amelting temperature no lower than the melting point of the fluoride,then the melted fluid is crystallized and the fluoride crystal thusobtained is cooled down in the temperature range from 1000° C. to 900°C. by the temperature decreasing rate in the range from 0.1 to 5° C./hr,whereby the maximum diameter d_(max) (cm) of the scattering bodiescontained in the fluoride crystal and the quantity n_(s) of thescattering bodies per 1 cm³ are sufficiently reduced. Therefore, it ispossible to obtain the optical member of the present invention asrepresented by the formula (1) easily and reliably by the manufacturingmethod of the present invention, and enhancement in a product yield andan increase in a diameter are thereby achieved.

[0029] Moreover, in the manufacturing method of the present invention,it is preferable that a position for carving the optical member out inthe carving step is selected based on a correlation among the maximumdiameter d_(max) (cm) of the scattering bodies obtained in advanceconcerning light with a specified wavelength, the quantity n_(s) of thescattering bodies per 1 cm³, and an amount of deterioration L ofinternal transmittance. In this way, it is possible to obtain theoptical member having desired internal transmittance easily andreliably.

[0030] Moreover, in the manufacturing method of the present invention,it is preferable that the fluoride powder for use therein has an averagegrain size no greater than 100 μm, and a proportion of grains havinggrain sizes in a range from 0.5 to 1.5 times of the average grain sizeaccounts for 50 weight % and above. It is possible to suppressgeneration of the scattering bodies by use of the above-describedfluoride powder, and n_(s) can be further reduced accordingly.

[0031] Moreover, in the manufacturing method of the present invention,it is preferable that concentrations of Cl, Br and I contained in thefluoride powder for use therein are severally below 0.1 ppm. In thisway, it is possible to further reduce the maximum diameter d_(max) (cm)of the scattering bodies contained in the fluoride crystal and thequantity n_(s) of the scattering bodies per 1 cm³.

[0032] Moreover, a projection exposure system of the present inventionincludes a reticle having a pattern, an illumination optical system forirradiating light having a wavelength of not more than 250 nm onto thereticle, and a projection optical system for forming an image of thepattern on the reticle irradiated by the illumination optical systemonto a wafer. Here, at least one of the illumination optical system andthe projection optical system includes an optical member made of afluoride crystal in which a maximum diameter d_(max) (cm) of scatteringbodies existing internally and a quantity n_(s) of the scattering bodiesper 1 cm³ satisfy a condition represented by the following formula (1):

0<d _(max) ² ×n _(s)<6.5×10⁻⁴ (cm⁻¹)  (1)

[0033] Deterioration in throughput of the optical system, deteriorationin contrast, occurrence of flares and ghosts and the like are suppressedsufficiently by applying the optical member of the present invention toat least one of the illumination optical system and the projectionoptical system. Accordingly, it is possible to achieve a sufficientlyhigh imaging performance in a case of using light having a wavelength ofnot more than 250 nm.

[0034] Moreover, in the projection exposure system of the presentinvention, it is preferable that the optical member has a diameter φ ofnot more than 200 mm. It is possible to further enhance the imagingperformance by increasing the diameter of the optical member asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a graph showing one example of a relation between aquantity n_(s) of scattering bodies per 1 cm³ of calcium fluoride andinternal transmittance thereof.

[0036]FIG. 2 is an explanatory view showing one example of a positionfor carving a conventional optical member out of a fluoride crystal.

[0037]FIG. 3 is an explanatory view showing one example of a positionfor carving an optical member of the present invention out of a fluoridecrystal.

[0038]FIG. 4 is a schematic constitutional view showing one preferredembodiment of a projection exposure system of the present invention.

[0039]FIG. 5 is a schematic constitutional view showing one preferredexample of a projection optical system according to the presentinvention.

Best mode for carrying out the Invention

[0040] In the following, description will be made in detail regarding apreferred embodiment of the present invention with reference to theaccompanying drawings as appropriate.

[0041] As described above, an optical member of the present invention isan optical member for photolithography used together with light having awavelength of not more than 250 nm. Here, the optical member is made ofa fluoride crystal, inwhich a maximum diameter d_(max) (cm) ofscattering bodies existing internally and a quantity n_(s) of thescattering bodies per 1 cm³ satisfy a condition represented by thefollowing formula (1):

0<d _(max) ² ×n _(s)<6.5×10⁻⁴ (Cm⁻¹)  (1)

[0042] Accordingly, the optical member possesses sufficient opticalcharacteristics (such as internal transmittance) with respect to thelight having the wavelength of not more than 250 nm.

[0043] Although fluoride crystal relevant to the present invention isnot particularly limited as long as d_(max) and n_(s) satisfy thecondition represented by the foregoing formula (1), a calcium fluoridecrystal, a lithium fluoride crystal, a barium fluoride crystal, astrontium fluoride crystal, a magnesium fluoride crystal, and the likeare cited to be more precise. It is preferable that these fluoridecrystals are single crystals.

[0044] In the following, description will be made concretely regardingmeasurement of internal transmittance with respect to light having awavelength at 193 nm and microscopic observation of scattering bodies ina case where calcium fluoride is the fluoride crystal, andquantification of a relation among the internal transmittance, andd_(max) and n_(s) of the scattering bodies based on results thereof.

[0045] First, concerning two types of calcium fluoride samples in whichscattering bodies exist on the inside with given distribution, testpieces severally having the diameter of 30 mm are collected from aplurality of positions different in densities of scattering bodies. Eachof these test pieces is subjected to mirror polishing so as to satisfythe condition where a distance (thickness) between two opposite andmutually parallel planes is 10 mm, parallelism is within 30 seconds, andsurface roughness RMS is within 5 Å.

[0046] Regarding the test pieces thus obtained, transmittance at thewavelength of 193 mm is measured by use of a spectrophotometer (such asCary 5 made by Varian, Inc.). The transmittance obtained here refers totransmittance including multiple reflection, which can be converted intointernal transmittance by use of the following formulae (4) and (5).

[0047] Specifically, if a refractive index is denoted by n, thenreflectance R on a surface of the test piece is represented by thefollowing formula (4). Meanwhile, a relation between the transmittanceT_(r) in consideration of a multiple reflection loss on the surface andthe internal transmittance Ti is represented by the following formula(5) by use of R:

R=(n−1)²/(n+1)²  (4)

T _(r)=(1−R)² ·Ti/(1−R ²·T_(i) ²)  (5)

[0048] In the formulae (4) and (5), it is possible to find the internaltransmittance T_(i) out of the transmittance T_(r) inclusive of themultiple reflection which is obtained by the measurement of thetransmittance, by means of applying the refractive index n=1.501 ofcalcium fluoride with respect to the light with the wavelength of 193nm. For example, if T_(r)=92.3% then T_(i)=100.0%, and if T_(r)=91.4%then T_(i)=99.0%.

[0049] Next, microscopic observation of the scattering bodies isperformed regarding positions where the measurement of the transmittanceof the test pieces took place. Note that the Japanese Optical GlassIndustrial Standards suggest that it is desirable to measure across-sectional area and a quantity by use of a sample in a size of 50ml or larger upon observation of foreign substances or bubbles. However,it is extremely difficult to perform measurement of spectraltransmittance by using such a large sample directly because of sizerestriction of a sample chamber or the like. Therefore, it is impossibleto find a relation among the transmittance, and the size and thequantity of the scattering bodies directly. Accordingly, the inventorsof the present invention have contemplated to find the relation betweenthe scattering body and the transmittance by means of measuring the sizeand the quantity of the scattering bodies in the region where thetransmittance of the test piece being subjected to the measurement ofthe transmittance was measured.

[0050] The maximum diameter d_(max) of the scattering bodies and aquantity n_(s) thereof per 1 cm³ can be found by the followingprocedures. Specifically, upon observation of the optical member with amicroscope (magnifications: 50, field of view: φ 4 mm), a stage mountingthe test piece is moved up and down, whereby the quantity of thescattering bodies observed within the view fields in an interval of 10mm from a front to a back of the test piece, and a maximum length ofcross sections thereof are measured. The measurement is carried out sixtimes in total while slightly changing the position of the test piece.Accordingly, it is possible to find d_(max) and n_(s) (which are averagevalues in both cases) out of the quantities of the counted scatteringbodies, areas of the fields of view and a moving distance (10 mm) of thestage. When such microscopic observation takes place, the diameter ofthe scattering bodies and the quantity thereof normally show differentvalues depending on each ingot.

[0051]FIG. 1 is a graph showing one example of a relation between thequantity n_(s) of the scattering bodies per 1 cm³ of fluorite andinternal transmittance, in which the axis of abscissas indicates n_(s)and the axis of ordinates indicates the internal transmittance T_(i).Marks o in the drawing represent plotted points of the internaltransmittance T_(i) with respect to the quantity n_(s) of the scatteringbodies regarding calcium fluoride in which the maximum diameter d_(max)of the scattering bodies is 2.0×10⁻³ cm (20 μm), and a line la is anapproximated curve for those points. Meanwhile, marks * in the drawingrepresent plotted points of the internal transmittance T_(i) withrespect to the quantity n_(s) of the scattering bodies regarding calciumfluoride in which the maximum diameter d_(max) of the scattering bodiesis 4.0×10⁻³ cm (40 μm), and a line 1 b is an approximated curve forthose points.

[0052] As plotted in the graph, when the maximum diameter d_(max) of thescattering bodies is constant, a linear relation is observed between thequantity n_(s) of the scattering bodies and the internal transmittanceT_(i). The inventors of the present invention have further examined on acorrelation among the maximum diameter d_(max) as well as the quantityper 1 Cm³ Of the scattering bodies and an amount of deterioration L ofthe internal transmittance T_(i) per 1 cm, and have found out acorrelation represented by the following formula (6) as a consequence:

L=d _(max) ² ×n _(s) ×C ₁  (6)

[0053] (in the formula (6), L denotes the amount of deterioration (%/cm)of the internal transmittance per 1 cm; C₁ denotes a coefficient when athickness of a sample is 1 cm; and d_(max) as well as n_(s) severallydenote the definition contents identical to those in the formula (1)).

[0054] In other words, regarding calcium fluoride which shows differentcorrelations between n_(s) and T_(i) as shown in the straight lines 1 aand 1 b in FIG. 1, the amount of deterioration L of the internaltransmittance can be expressed in one formula as the above-describedformula (6) by considering the maximum diameter d_(max) of thescattering bodies. Here, the coefficient C₁ is 3.1 when the wavelengthof the light is 193 nm.

[0055] According to the formula (6), the internal transmittance T_(i)exceeds 99.8%/cm when d_(max) and n_(s) satisfy a condition representedby the following formula (7):

0.998<1−d _(max) ² ×n _(s)×3.1 (cm⁻¹)  (7)

[0056] in other words, when d_(max) and n_(s) satisfy a conditionrepresented by the following formula (8):

d _(max) ² ×n _(s)<6.5×10⁻⁴ (cm⁻¹)  (8)

[0057] Therefore, even in the case of the optical member made of calciumfluoride containing the scattering bodies therein, it is still possibleto obtain excellent optical performance when used in an optical systemfor photolithography, provided, that the maximum diameter d_(max) andthe quantity n_(s) of the scattering bodies satisfy the conditionrepresented by the formula (8), that is, the formula (1).

[0058] To be more precise, from the formula (8) i.e. the formula (1),the internal transmittance of 99.8%/cm and above can be achieved if thequantity n_(s) of the scattering bodies per 1 cm³ is no more than 160 inthe case where the maximum diameter of the scattering bodies presenttherein is 20 μm or smaller, or if the quantity n_(s) of the scatteringbodies per 1 cm³ is no more than 40 in the case where the maximumdiameter of the scattering bodies present therein is 40 μm or smaller.

[0059] Meanwhile, in the case where the internal transmittance T_(i)should only satisfy 99.5%/cm or greater, then d_(max) and n_(s) shouldonly satisfy a condition represented as the following formula (9):

0.995<1−d _(max) ² ×n _(s)×3.1 (cm⁻¹)  (9)

[0060] in other words, d_(max) and n_(s) should only satisfy a conditionrepresented by the following formula (10):

d _(max) ² ×n _(s)<1.6×10⁻³ (cm⁻¹)  (10)

[0061] Although description has been made above regarding the example inthe case of the ArF excimer laser beam (193-nm wavelength), the opticalmember of the present invention is also effective when used togetherwith other light having a wavelength of not more than 250 nm, such as aKrF excimer laser beam (248-nm wavelength) or an F₂ laser beam (157-nmwavelength).

[0062] Next, description will be made regarding a method ofmanufacturing an optical member of the present invention.

[0063] In the method of manufacturing an optical member of the presentinvention, firstly a mixture of fluoride powder and a scavenger ismelted at a melting temperature of a melting point for the fluoride andabove, then the melted fluid is crystallized and an obtained fluoridecrystal are further cooled down in a temperature range from 1000° C. to900° C. by a temperature decreasing rate in a range from 0.1 to 5° C./hr(a crystal growing step).

[0064] As for the fluoride powder for use in the manufacturing method ofthe present invention, calcium fluoride, lithium fluoride, bariumfluoride, strontium fluoride, magnesium fluoride, and the like arecited. It is preferable to remove impurity elements of metal or the likeas much as possible out of the fluoride powder of the above-mentionedingredients before subjecting the fluoride powder to a pre-processingstep. For example, it is preferable that concentrations of chlorine(Cl), bromine (Br) and iodine (I) contained in the raw material areseverally below 0.1 ppm. It is possible to further reduce the maximumdiameter d_(max) and the quantity n_(s) per 1 cm³ of the scatteringbodies contained in the fluoride crystal if the fluoride powder usedtherein satisfies the condition regarding the concentrations of Cl, Br,and I as described above. Moreover, it is preferable that concentrationsof cobalt (Co), cerium (Ce), lanthanum (La), yttrium (Y), iron (Fe) andlead (Pb) are severally below 0.5 ppm; concentrations of potassium (K),manganese (Mn), copper (Cu), nickel (Ni) and chromium (Cr) are severallybelow 0.1 ppm; concentrations of lithium (Li) and sodium (Na) areseverally below 0.2 ppm; a concentration of barium (Ba) is below 1.0ppm; and a concentration of strontium (Sr) is below 20 ppm.

[0065] Meanwhile, the scavenger has an effect of reducing concentrationsof impurities in the fluoride powder. To be more precise, those citedare metal fluorides such as lead fluoride, zinc fluoride or silverfluoride, gaseous fluorides such as fluorine (F₂) or carbontetrafluoride (tetrafluoromethane, CF₄), and fluorine-containing organiccompounds such as PTFE (polytetrafluoroethylene). An amount of additionof the scavenger is not particularly limited; however, it is preferableto be in a range from 0.1 to 10 mol % with respect to the fluoridepowder raw material in the case of a metal fluoride, for example. If thefluoride powder raw material is calcium fluoride and the scavenger islead fluoride, for example, then it is preferable to use 0.3 to 35 g oflead fluoride with respect to 100 g of calcium fluoride.

[0066] In the event of performing the crystal growing step, it ispreferable to subject the mixture of the fluoride powder and thescavenger to a given pre-process in advance so as to remove theimpurities in the mixture and to increase a bulk density thereof. Forexample, it is possible to attempt homogenization of viscosity andcomponents of a melted fluid obtained by filling the mixture of thefluoride powder and the scavenger into a crucible and then by heatingand melting the mixture inside a given pre-processing apparatus. In thisevent, it is preferable to maintain the crucible and the inside of thepre-processing apparatus as clean as possible, and it is preferable todischarge air out of the apparatus before heating when the raw materialis introduced. Moreover, it is preferable that the pre-processing stepis performed in a clean room which maintains a cleanness factor betterthan class 1,000,000.

[0067] The processing temperature and retention time in thepre-processing step vary depending on the types of the fluoride powderand the scavenger. In the case where the fluoride raw material iscalcium fluoride, for example, the temperature is preferably set in arange from 1420° C. to 1500° C., and the retention time is preferablyset in a range from 12 to 36 hours. A reaction between the fluoridepowder and the scavenger is promoted by performing the pre-process underthe above-described condition, and the viscosity and the components ofthe melted fluid can be sufficiently homogenized. In this event, it ispreferable that a temperature increasing rate in the process of heatingup to the above-mentioned temperature is set in a range from 1 to 15°C./hr. Moreover, it is preferable that the temperature rise is stoppedtemporarily and held at a given temperature (preferably in a range from150° C. to 350° C.) because the impurities such as water or carbondioxide can be removed by evaporation.

[0068] In the pre-processing step, the melted fluid homogenized in theviscosity and the components is cooled down in accordance with a giventemperature decreasing rate (preferably in a range from 10 to 30°C./min). Thereafter, the pre-processing step is finished upon completionof crystallization of the melted fluid, and the fluoride crystal thusobtained is subjected to the crystal growing process as raw materialbulk.

[0069] The crystal growing step according to the present invention canbe performed by the vertical Bridgman method, for example. Specifically,either the mixture of the fluoride powder and the scavenger or thefluoride crystal obtained in the preprocessing step is put into acrucible and then introduced into a crystal growth apparatus (a crystalgrowth furnace), and the fluoride crystal is melted at a meltingtemperature equal or above a melting point for the fluoride crystal(which is 1420° C. and above in the case of calcium fluoride).Thereafter, the crucible is pulled down from the furnace at a given ratefor pulling down, whereby the melted fluid is crystallized.

[0070] Here, the melting temperature in the crystal growing step refersto the temperature of the melting point of the fluoride crystal orhigher as described above, which is preferably set in a range from 1420°C. to 1500° C. in the case of calcium fluoride. Moreover, the retentiontime at the melting temperature is preferably set in a range from 8 to24 hours.

[0071] Moreover, the rate for pulling down the crucible is preferablyset in a range from 0.1 to 5 mm/hr. If the rate for pulling down exceedsthe upper limit, the maximum diameter d_(max) and the quantity n_(s) per1 cm³ of the scattering bodies in the fluoride crystal tend to increase.On the contrary, manufacturing efficiency tends to drop if the ratefalls below the lower limit.

[0072] Crystallization is completed normally in a range from 1200° C. to1350° C. by pulling down the crucible as described above.

[0073] The obtained fluoride crystal is slowly cooled down to a giventemperature (preferably in a range from 400° C. to 750° C.). Here, it isessential that the temperature decreasing rate in the event of coolingdown from 1000° C. to 900° C. is set in a range from 0.1 to 5° C./hr. Ifthe temperature decreasing rate in the temperature range exceeds theabove-mentioned upper limit, the maximum diameter d_(max) and thequantity n_(s) per 1 cm³ Of the scattering bodies in the fluoridecrystal are increased, then, it is extremely difficult to achieveenhancement in a yield and an increase in a diameter upon carving outthe optical member of the present invention. Moreover, if thetemperature decreasing rate is too fast as mentioned above, the fluoridecrystal may break up easily because of development of cracks or thelike, and homogeneity in the refractive index is deteriorated as well.On the contrary, productivity becomes inadequate if the temperaturedecreasing rate falls below the lower limit. Such a slow cooling stepcan be performed by means of setting the crucible once pulled down backto the crystal growth apparatus again while controlling the temperatureinside the apparatus, for example.

[0074] Meanwhile, it is preferable that the temperature decreasing ratein the event of cooling down from the point of completion of thecrystallization to 1000° C. is set in a range from 1 to 15° C./hr. Ifthe temperature decreasing rate in the above-mentioned temperature rangeexceeds the upper limit, the fluoride crystal may break up easilybecause of development of cracks or the like, and homogeneity in therefractive index tends to be deteriorated as well. On the contrary,operability tends to be degraded if the temperature decreasing ratefalls below the lower limit. For example, in the case of slowly coolingdown the crystal ingot once pulled down by means of lifting it up closerto the central portion of the crystal growth apparatus, such rapidcooling down immediately after the crystallization is extremelydifficult due to the structure of the furnace.

[0075] Furthermore, after the temperature of the fluoride crystalreached 900° C., it is possible to continue slow cooling with the sametemperature decreasing rate or to perform multistage slow cooling withdifferent temperature decreasing rates. However, in either case, thetemperature decreasing rate in a temperature range from 900° C. to 750°C. is preferably set in a range from 0.1 to 5° C./hr (more preferably ina range from 0.2 to 2° C./hr), and the temperature decreasing rate from750° C. down to completion of slow cooling is preferably set in a rangefrom 1.0 to 15° C./hr. By slow cooling with the temperature decreasingrates as described above, it is possible to further enhance the effectof reducing the size and the quantity of the scattering bodies in thefluoride crystal, and to enhance the effect of preventing development ofcracks and deterioration of homogeneity in the refractive index as well.

[0076] Note that it is preferable to perform removal of the fluoridecrystal out of the crucible after the pre-processing step and filling ofthe fluoride crystal into the crucible in the growing step in a cleanroom which maintains a cleanness factor better than class 1,000,000 assimilar to the pre-processing step in order to avoid interfusion ofimpurity elements or dust. It is also possible to prevent an increase ofthe scattering bodies attributable to disturbance from the environmentoutside the apparatus by means of disposing the crystal growth furnacein the clean room, which maintains the cleanness factor better thanclass 1,000,000, together with an earthquake-resistant structure, and bycontrolling the temperature inside the clean room to a given temperature(at 25±1° C., for example)

[0077] Moreover, although description has been made herein regarding thecase of performing the pre-processing step and the crystal growing stepseparately, it is not always necessary to perform the pre-processingstep and the crystal growing step separately in the manufacturing methodof the present invention. For example, it is also possible to melt themixture of the fluoride powder and the scavenger in the pre-processingstep and then grow the crystal by pulling down the crucible containingthe melted fluid while controlling the above-described temperatures orthe temperature decreasing rates.

[0078] The optical member of the present invention can be obtained bycarving a material of a desired shape out of the fluoride crystal (theingot) thus obtained.

[0079] A position for carving the optical member out of the fluoridecrystal is selected based on measured values of the maximum diameterd_(max) and the quantity n_(s) per 1 cm³ of the scattering bodiesobtained by microscopic observation. Here, as the maximum diameterd_(max) and the quantity n_(s) per 1 cm³ of the scattering bodies aresufficiently reduced in the fluoride crystal obtained by themanufacturing method of the present invention, it is possible to enhancethe product yield and to increase the diameter. Specifically, as shownin FIG. 3, d_(max) and n_(s) at a portion 2 a containing the scatteringbodies satisfy the condition represented by the aforementioned formula(1) in the case of a fluoride crystal 2 obtained by the manufacturingmethod of the present invention. Accordingly, an obtained optical member4 possesses sufficiently high optical performance if it is carved outwhile including the portion 2 a so as to enhance the product yield andto increase the diameter.

[0080] Meanwhile, in the case of carving an optical member havingcertain internal transmittance with respect to light having a specificwavelength (such as light having a shorter wavelength than an F₂ laserbeam), then the coefficient C₁ in the formula (6) with respect to thelight is determined in advance. Thereafter, internal transmittance at acarving position can be estimated without measuring the internaltransmittance but based on the formula (6) applying the C₁ value and themeasured values of the maximum diameter d_(max) and the quantity n_(s)per 1 cm³ of the scattering bodies obtained by microscopic observationof the ingot. In this way, it is possible to obtain the optical memberwith the desired optical performance easily and reliably.

[0081] Moreover, in the present invention, the optical member carved outof the fluoride crystal may be subjected to processes such as annealingor mirror polishing when necessary. In particular, as the atmosphere isfluorinated if annealing is performed in the presence of a fluorinatingagent such as ammonium bifluoride, PTFE, F₂ and CF₄. Accordingly, it ispossible to prevent oxidation of the fluoride crystal and to furtherreduce d_(max) and n_(s). Alternatively, a similar effect can be alsoobtained by replacing the inside of an annealing furnace with inert gassuch as argon instead of using the fluorinating agent. Although aprocessing temperature in annealing varies depending on the type of thefluoride crystal, it is preferable to set the temperature in a rangefrom 1000° C. to 1200° C. in the case of calcium fluoride, for example.

[0082] As described above, according to the manufacturing method of thepresent invention, the maximum diameter d_(max) and the quantity n_(s)per 1 cm³ of the scattering bodies contained in the fluoride crystal canbe sufficiently reduced. As a result, it is possible to achieveenhancement in the yield and an increase in the diameter upon carvingthe optical member of the present invention out of the fluoride crystal.

[0083] Next, description will be made regarding a projection exposuresystem of the present invention.

[0084]FIG. 4 is a schematic constitutional view showing one preferredembodiment of a projection exposure system of the present invention. InFIG. 4, reference numeral 11 denotes a light source, reference numeral12 denoted an illumination optical system, reference numeral 12 adenotes an alignment optical system, reference numeral 12 b denotes anillumination lens, reference numeral 13 denotes a reticle, referencenumeral 14 denotes a reticle stage, reference numeral 15 denotes aprojection optical system, reference numeral 15 a denotes an aperture,reference numeral 15 b denotes a projection lens, reference numeral 16denotes a wafer, reference numeral 17 denotes a wafer stage, referencenumeral 18 denotes a reticle changer system, reference numeral 19denotes a wafer stage control system, and reference numeral 20 denotes amain control unit.

[0085] As for the light source 11, a KrF excimer laser, an ArF excimerlaser, an F₂ laser and the like are usable, for example. The lightemitted out of the light source 11 forms uniform illumination light bythe illumination lens in the illumination optical system 12, andilluminates a surface of the reticle 13 mounted on the reticle stage 14.

[0086] The light transmitted through a pattern provided on the reticle13 further passes through the apertures 15 a of the projection opticalsystem 15, and then form an image of the pattern of the reticle 13 on asurface of the wafer 16. The illumination optical system 12 is providedwith the alignment optical system 12 a for adjusting a relative positionbetween the reticle 13 and the wafer 16. Moreover, the reticle changersystem 18 and the wafer stage control system 19 are provided asauxiliary equipment, and the entire system is controlled by the maincontrol unit 20.

[0087] As described above, in the projection exposure system of thepresent invention, the light emitted out of the light source 11 passesthrough numerous optical members including the alignment optical system12 a, the illumination lens 12 b, the projection lens 15 b and the like.Here, sufficiently high optical performances with respect to the lighthaving a wavelength of not more than 250 nm can be achieved by using theoptical member of the present invention as the optical member for atleast one of the illumination optical system 12 and the projectionoptical system 15, or further for the alignment optical system 12 a.

[0088] Meanwhile, the illumination optical system 12 or the projectionoptical system 15 may be composed by including optical members (lenses)made of calcium fluoride crystals which do not satisfy the conditionrepresented by the formula (1). However, it is preferable that anoptical path length of the optical member of the present inventionaccounts for 10% and above (more preferably 50% and above) of anaggregate sum of optical path lengths of the optical members made ofcalcium fluoride. By constructing the optical systems so as to satisfythe foregoing condition, throughput of the entire optical systems arefurther enhanced, also, deterioration in contrast as well as occurrenceof flares and ghosts can be prevented more reliably.

[0089] Now, description will be made further in detail regarding theprojection optical system 15 composed by use of the optical member ofthe present invention with reference to FIG. 5. FIG. 5 is a schematicconstitutional view showing one preferred example of the projectionoptical system 15 according to the present invention. In the orderstarting from a reticle R side as a first object, the projection opticalsystem 15 sequentially includes a first lens group G1 of positive power,a second lens group G2 of positive power, a third lens group G3 ofnegative power, a fourth lens group G4 of positive power, a fifth lensgroup G5 of negative power, and a sixth lens group G6 of positive power.Moreover, the object side (the reticle R side) and an image side (awafer W side) are made almost telecentric, and the projection opticalsystem possesses reductive power. Moreover, N. A. of this projectionoptical system is 0.6 and projected magnification is ¼.

[0090] In this projection optical system, six lenses L₄₅, L₄₆, L₆₃, L₆₅,L₆₆ and L₆₇ adopt ones made of fluoride crystals, and the rest of thelenses adopt ones made of fused silica. In this case, it is preferablethat the optical path length of the optical member of the presentinvention accounts for 10% and above (more preferably 50% and above) ofan aggregate sum of optical path lengths of the six lenses made ofcalcium fluoride. It is particularly preferable that all the six lensesare the optical members of the present invention.

EXAMPLES

[0091] The present invention will be hereinafter described moreconcretely based on examples and comparative examples. However, it is tobe noted that the present invention is not limited to the followingexamples in any respect.

Example 1

[0092] (Fabrication of Optical Member)

[0093] Impurity elements such as metal were removed to the utmost from50 kg of calcium fluoride powder (average grain size: 40 μm, aproportion of grains having sizes no smaller than 0.5 times and nolarger than 1.5 times of the average grain size: 57%, concentrations ofimpurities: Cl<0.1 ppm, Br<0.1 ppm, I<0.1 ppm, Co<0.05 ppm, Ce<0.05 ppm,La<0.05 ppm, Y<0.05 ppm, Mn<0.1 ppm, Cu<0.1 ppm, Ni<0.1 ppm, K<0.1 ppm,Cr<0.1 ppm, Li<0.2 ppm, Na <0.2 ppm, Pb<0.5 ppm, Fe<0.5 ppm, Ba<1.0 ppm,Sr<20 ppm). 1.6 kg of lead fluoride (about 1 mol %) was added as ascavenger thereto and agitation was satisfactorily performed. Themixture was put into a crucible made of carbon, which was held in aclean condition by rinsing with calcium fluoride powder. Then thecrucible was introduced into a pre-processing apparatus which wascleaned and held in a clean condition. After the inside of the apparatuswas pumped out to vacuum, the temperature was increased and maintainedat 300° C. for a given time period, whereby impurities such as water orcarbon dioxide were removed by evaporation. Next, the temperature wasincreased slowly so as to promote a sufficient reaction between thecalcium fluoride powder and the scavenger. The calcium fluoride wasmelted at 1420° C., and the temperature was maintained at the same levelfor 24 hours to promote homogenization of viscosity and components ofthe melted fluid. Thereafter, the temperature inside the apparatus wasdecreased to crystallize the melted fluid.

[0094] Next, the step of growing a calcium fluoride crystal by thevertical Bridgman method was carried out in accordance with thefollowing procedures.

[0095] First, the calcium fluoride crystal obtained in thepre-processing step was used as raw material bulk and put into acrucible made of carbon, which was held in a clean condition by rinsingwith calcium fluoride powder. Then the crucible was introduced into acrystal growth apparatus which was cleaned and held in a cleancondition. The inside of the apparatus was pumped out to vacuum and thenthe temperature was slowly increased under control by heating with aheater. When the temperature inside the apparatus reached 1420° C., thetemperature was maintained for 24 hours to promote homogenization of themelted fluid. Thereafter, the crucible was pulled down by a pulling downrate of 1 mm/hr to promote crystallization.

[0096] After the melted fluid was entirely crystallized, the cruciblewas placed back into the crystal growth apparatus. The temperatureinside the apparatus was slowly cooled down to room temperature undercontrol, whereby a crystal ingot of the calcium fluoride was obtained.Temperature decreasing rates upon slow cooling were set severally as: 3°C./hr until reaching 1000° C., 1° C./hr from 1000° C. down to 900° C.,and 5° C./hr from 900° C. down to 500° C. The ingot was further cooleddown from 500° C. to the room temperature by leaving the ingotuncontrolled in the furnace.

[0097] The inside of the ingot thus obtained was observed under aspotlight. Although scattered light from scattering bodies was observed,the scattered light was less in a lower half of the ingot than an upperhalf thereof.

[0098] A material of 200-mm diameter and 50-mm thickness was carved outof the lower half of the ingot and introduced into an annealing furnacetogether with a fluorinating agent (ammonium bifluoride). After theinside of the furnace was pumped out to vacuum, the temperature wasincreased to 1050° C. by a temperature increasing rate of 50° C./hr andmaintained at the same level for 24 hours. Thereafter, the material wasslowly cooled down to 900° C. by a temperature decreasing rate of 2°C./hr and further down to room temperature by a temperature decreasingrate of 5° C./hr. A targeted optical member was thereby obtained.

[0099] A maximum diameter d_(max) and a quantity n_(s) per 1 cm³ of thescattering bodies contained in the obtained optical member, internaltransmittance with respect to light having a wavelength at 193 nm (theArF excimer laser beam), an amount of birefringence in the direction ofthe optical axis or the diametrical direction, a refractive indexdifference and an amount of transmittance deterioration are shown onTable 1. The value of d_(max) ²×n_(s) in the optical member obtained inthis example was

(1.8×10⁻³)²×150=4.9×10⁻⁴<6.5×10⁻⁴ (cm⁻¹)

[0100] and was therefore confirmed to satisfy the condition representedby the formula (1). Meanwhile, the internal transmittance with respectto the light having the wavelength at 193 nm showed a high value of99.9%/cm.

Example 2

[0101] 1.6 kg of lead fluoride (about 1 mol %) was added as a scavengerto 50 kg of calcium fluoride powder similar to example 1 and agitationwas satisfactorily performed. The mixture was put into a crucible madeof carbon, which was held in a clean condition by rinsing with calciumfluoride powder. Then the crucible was introduced into a pre-processingapparatus which was cleaned and held in a clean condition. Here, theabove-mentioned operations were performed inside a clean room of class10,000 in order to avoid interfusion of impurity elements or dust in theevent of mixing and agitation and in the event of filling the rawmaterial.

[0102] After the inside of the apparatus was pumped out to vacuum, thetemperature was maintained at 300° C. for a given time period, wherebyimpurities such as water or carbon dioxide were removed by evaporation.Next, the temperature was increased slowly so as to promote a sufficientreaction between the calcium fluoride powder and the scavenger. Thecalcium fluoride was melted at 1420° C., and the temperature wasmaintained at the same level for 24 hours to promote homogenization ofviscosity and components of the melted fluid. Thereafter, thetemperature inside the apparatus was decreased to crystallize the meltedfluid.

[0103] Next, the step of growing a calcium fluoride crystal by thevertical Bridgman method was carried out in accordance with thefollowing procedures.

[0104] First, the calcium fluoride crystal obtained in thepre-processing step was used as raw material bulk and put into acrucible made of carbon, which was held in a clean condition by rinsingwith calcium fluoride powder. Then the crucible was introduced into acrystal growth apparatus which was cleaned and held in a cleancondition. Here, the operations of removing the fluoride crystal out ofthe crucible after the pre-processing step and filling of the fluoridecrystal into the crucible in the growing step in a clean room wereperformed inside the clean room of class 10,000 in order to avoidinterfusion of impurity elements or dust. Moreover, the crystal growthapparatus is provided with an earthquake-resistant structure anddisposed inside the clean room of class 100,000, and the temperatureinside the clean room was controlled to 25±1° C., whereby an increase ofthe scattering bodies attributable to disturbance from the environmentoutside the apparatus was prevented.

[0105] After the inside of a sample chamber of this crystal growthapparatus was pumped out to vacuum, the temperature was slowly increasedunder control by heating with a heater. When the temperature reached1420° C., the temperature was maintained for 24 hours to promotehomogenization of the melted fluid. Thereafter, the crucible was pulleddown by a pulling down rate of 0.3 mm/hr to promote crystallization.

[0106] After the melted fluid was entirely crystallized, the cruciblewas placed back into the crystal growth apparatus. The temperatureinside the apparatus was slowly cooled down to room temperature undercontrol, whereby a crystal ingot of the calcium fluoride was obtained.Temperature decreasing rates upon slow cooling were set severally as: 3°C./hr until reaching 1000° C., 1° C./hr from 1000° C. down to 900° C.,and 5° C./hr from 900° C. down to 500° C. The ingot was further cooleddown from 500° C. to the room temperature by leaving the ingotuncontrolled in the furnace.

[0107] The inside of the ingot thus obtained was observed under aspotlight. Although scattered light from scattering bodies was observedin an upper part of the ingot, the scattered light was not observed in alower half of the ingot.

[0108] Materials each having 200-mm diameter and 50-mm thickness werecarved severally out of the upper half and the lower half of this ingotand introduced into an annealing furnace together with a fluorinatingagent. After the inside of the furnace was pumped out to vacuum, thetemperature was increased to 1050° C. by a temperature increasing rateof 50° C./hr and maintained at the same level for 24 hours. Thereafter,the materials were slowly cooled down to 900° C. by a temperaturedecreasing rate of 2° C./hr and further down to room temperature by atemperature decreasing rate of 5° C./hr. Targeted optical members werethereby obtained.

[0109] A maximum diameter d_(max) and a quantity n_(s) per 1 cm³ of thescattering bodies contained in the obtained optical members, internaltransmittance with respect to light having a wavelength at 193 nm (theArF excimer laser beam), amounts of birefringence in the direction ofthe optical axis or the diametrical direction, refractive indexdifferences and amounts of transmittance deterioration are shown onTable 1.

[0110] No scattering body was observed in the case of the optical membercarved out of the upper half of the ingot, and the internaltransmittance, the amounts of birefringence in the direction of theoptical axis or the diametrical direction, the refractive indexdifference and the amount of transmittance deterioration thereof werealso confirmed to be excellent. Moreover, the value of d_(max) ²×n_(s)in the optical member carved out of the lower half was

(3.6×10⁻³)²×27=3.5×10⁻⁴<6.5×10⁻⁴ (cm⁻¹)

[0111] and was therefore confirmed to satisfy the condition representedby the formula (1). Meanwhile, the internal transmittance with respectto the light having the wavelength at 193 nm showed a high value of99.9%/cm.

Comparative Example 1

[0112] An optical member was fabricated as similar to example 1, exceptthat the environment of preservation of the crucible, the pre-processingapparatus and the crystal growth apparatus was not administered, andthat the temperature decreasing rate from the point of crystallizationdown to 500° C. was set to 30° C./hr in the crystallizing step.

[0113] A maximum diameter d_(max) and a quantity n_(s) per 1 cm³ of thescattering bodies contained in the obtained optical member, internaltransmittance with respect to light having a wavelength at 193 nm (theArF excimer laser beam), amounts of birefringence in the direction ofthe optical axis or the diametrical direction, a refractive indexdifference and an amount of transmittance deterioration are shown onTable 1.

[0114] The value of d_(max) ²×n_(s) in the optical member obtained inthis comparative example was

(5.7×10⁻³)2×30=9.7×10⁻⁴>6.5×10⁻⁴ (Cm⁻¹)

[0115] and was not therefore confirmed to satisfy the conditionrepresented by the formula (1). Meanwhile, the internal transmittancewith respect to the light having the wavelength at 193 nm was 99.7%/cm.

Comparative Example 2

[0116] An optical member was fabricated as similar to example 1, exceptthat the temperature decreasing rate from the point of crystallizationdown to 500° C. was set to 30° C./hr in the crystallizing step.

[0117] A maximum diameter d_(max) and a quantity n_(s) per 1 cm³ of thescattering bodies contained in the obtained optical member, internaltransmittance with respect to light having a wavelength at 193 nm (theArF excimer laser beam), amounts of birefringence in the direction ofthe optical axis or the diametrical direction, a refractive indexdifference and an amount of transmittance deterioration are shown onTable 1.

[0118] The value of d_(max) ²×n_(s) in the optical member obtained inthis comparative example was

(1.8×10⁻³)²×290=9.4×10⁻⁴>6.5×10⁻⁴ (cm⁻¹)

[0119] and was not therefore confirmed to satisfy the conditionrepresented by the formula (1). Meanwhile, the internal transmittancewith respect to the light having the wavelength at 193 nm was 99.7%/cm.TABLE 1 Compara- Compara- Example Example tive tive 1 2 Example 1Example 2 Scattering Maximum 18 36 57 18 body diameter d_(max) (μm)Quantity n_(s) 150 27 30 290 Internal 99.9 99.9 99.7 99.7 transmittance(%/cm) Bire- Optical axis 0.9 1.9 1.7 1.4 fringence direction (nm/cm)Diametrical 2.3 4.4 3.9 3.8 direction Refractive index 1.2 × 10⁻⁶ 1.8 ×10⁻⁶ 1.8 × 10⁻⁶ 1.5 × 10⁻⁶ difference Transmittance 0.6 0.2 0.5 1.2deterioraticn (%/cm) Overall judgment Good Good NG NG

[0120] (Fabrication of projection exposure system)

Example 3

[0121] Among the lenses constituting the projection optical system shownin FIG. 5, the optical member of example 1 was applied to six lensesL₄5, L₄₆, L₆₃, L₆₅, L₆₆ and L₆₇, and lenses made of fused silica(internal transmittance including loss by scattering: about 99.8%) wereapplied to the rest of the lenses. Accordingly, a projection exposuresystem shown in FIG. 4 was fabricated.

[0122] As a result of evaluation of an imaging performance of thisprojection exposure system, desired throughput (135 sheets/hr) wasachieved in the case of φ200-mm wafers. Moreover, flares and ghostsobserved therein accounted for about 1% as light noises, which werealmost ignorable in practical use.

Comparative Example 3

[0123] Another projection exposure system was fabricated as similar toexample 3, except that the optical member obtained in comparativeexample 1 was applied instead of the optical member of example 1.

[0124] As a result of evaluation of an imaging performance of thisprojection exposure system, overall transmittance of the projectionoptical system was higher than example 3 by about 5%, and the desiredthroughput was not achieved. Moreover, flares and ghosts observedtherein accounted for about 7% as light noises.

INDUSTRIAL APPLICABILITY

[0125] As described above, according to the present invention, providedare an optical member having sufficiently high optical characteristics(such as internal transmittance) with respect to light with a wavelengthat 250 nm or less and being capable of enhancing a yield and increasinga diameter upon carving out of a fluoride crystalline material, a methodof manufacturing the same, and a projection exposure system using theoptical member. Therefore, the present invention realizes a high-levelimaging performance in micro-fabrication technology on a wafer.

What is claimed is:
 1. An optical member for photolithography usedtogether with light having a wavelength of not more than 250 nm,consisting essentially of a fluoride crystal in which a maximum diameterd_(max) (cm) of scattering bodies existing internally and a quantityn_(s) of the scattering bodies per 1 cm³ satisfy a condition representedby following formula (1), 0<d _(max) ² ×n _(s)<6.5×10⁻⁴ (cm¹)  (1) 2.The optical member according to claim 1, wherein the maximum diameterd_(max) of the scattering bodies is not more than 2.0×10⁻² cm, and thequantity n_(s) of the scattering bodies per 1 cm³ is not more than 160.3. The optical member according to claim 1, wherein the maximum diameterd_(max) of the scattering bodies is not more than 4.0×10⁻² cm, and thequantity n_(s) of the scattering bodies per 1 cm³ is not more than 40.4. The optical member according to claim 1, wherein a diameter φ thereofis not less than 200 mm.
 5. The optical member according to claim 1,wherein an amount of birefringence in a direction of an optical axis isnot more than 2 nm/cm.
 6. The optical member according to claim 1,wherein an amount of birefringence in a diametrical direction is notmore than 5 nm/cm.
 7. The optical member according to claim 1, wherein arefractive index difference Δn inside the member is not more than2×10⁻⁶.
 8. The optical member according to claim 1, wherein an amount oftransmittance deterioration upon irradiation of an ArF excimer laserbeam having energy density of 50 mJ/cm²/pulse is not more than 2.0%/cm.9. A method of manufacturing an optical member of comprising: a crystalgrowing step of melting a mixture of fluoride powder and a scavenger ata melting temperature of a melting point for the fluoride and above andthen crystallizing the melted fluid and further cooling down an obtainedfluoride crystal in a temperature range from 1000° C. to 900° C. by atemperature decreasing rate in a range from 0.1 to 5° C./hr; and acarving step of carving an optical member out of the fluoride crystalobtained in the crystal growing step such that the optical member ismade of a fluoride crystal in which a maximum diameter d_(max) ofscattering bodies existing internally and a quantity n_(s) of thescattering bodies per 1 cm³ satisfy a condition represented by thefollowing formula (1): 0<d _(max) ×n _(s)<6.5×10⁻⁴ (cm¹)  (1)
 10. Themethod of manufacturing an optical member according to claim 9, whereina position for carving the optical member out in the carving step isselected based on a correlation among the maximum diameter d_(max) (cm)of the scattering bodies obtained in advance concerning light with aspecified wavelength, the quantity n_(s) of the scattering bodies per 1cm³, and an amount of deterioration L of internal transmittance.
 11. Themethod of manufacturing an optical member according to claim 9, whereinthe fluoride powder to be used has an average grain size of 100 μm andbelow, and a proportion of grains having grain sizes in a range from 0.5to 1.5 times of the average grain size accounts for 50 weight % andabove.
 12. The method of manufacturing an optical member according toclaim 9, wherein the fluoride powder to be used has concentrations ofCl, Br and I severally below 0.1 ppm.
 13. A projection exposure systemcomprising: a reticle having a pattern; an illumination optical systemfor irradiating light having a wavelength of not more than 250 nm ontothe reticle; and a projection optical system for forming an image of thepattern on the reticle irradiated by the illumination optical systemonto a wafer, wherein at least one of the illumination optical systemand the projection optical system includes an optical member made of afluoride crystal, in which a maximum diameter d_(max) of scatteringbodies existing internally and a quantity n_(s) of the scattering bodiesper 1 cm³ satisfy a condition represented by following formula (1): 0<d_(max) ² ×n _(s)<6.5×10⁻⁴ (cm⁻¹)  (1)
 14. The projection exposure systemaccording to claim 13, wherein the optical member has a diameter φ ofnot less than 200 mm.